1. Technical Field
The present invention relates generally to electrical test probes and contacts for interconnecting integrated circuit (IC) devices and electronic components and systems, and specifically, to miniature high speed circuit probes and contacts having a low inductance and a low resistance. Applications include a test socket for connecting a ball grid array (BGA) IC package or a land grid array (LGA) IC package to a test system board; a socket for connecting an electronic device to a next level electronic hardware, battery and charging contacts, or other applications requiring low inductance and low contact resistance. The sockets can be used in an interposer configuration which provides separable interfaces for two oppositely disposed components having corresponding patterns (e.g., LGA to LGA or BGA to LGA) or, alternatively, the contacts can be permanently attached to a printed circuit board (PCB) or a system motherboard and provide a separable interface for an electronic device.
2. Background Art
Sockets and connectors are necessary for separably interconnecting IC devices such as bare chips, IC packages, and electronic modules, to other devices, motherboards, test systems, and alike. A typical application is an electrical socket for connecting an IC device to the next level of electronic hardware or to a test unit. The contacts are positioned and maintained in a required array by an insulating housing which has contact receiving cavities, disposed in a pattern corresponding to the array of input/output (I/O) terminals in each of the mated devices. The socket is usually interposed between the mated devices and provides a separable interface to each device. This allows the socket to be attached by clamping to one of the components, such as a PCB, typically by clamping with a standard hardware. The clamping preloads each contact against a respective I/O terminal on the PCB. The other end of the contact extends from the housing and is adapted to connect to the corresponding I/O terminal of the mating device.
A typical contact probe consists of a hollow barrel, a spring, and two plungers. The spring and the body portions of the plungers are guided in the barrel and are retained in the barrel which is rolled or crimped at both ends. In order to reduce the contact probe resistance and inductance, the probes typically rely on the plungers randomly tilting (i.e., deviating from axial alignment with the barrel) and electrically shorting to the barrel, so that most of the current could bypass the spring and flow through the barrel. This creates a conductive shunt which significantly lowers the overall contact resistance of the probe and the parasitic electrical effects of the coil spring. The plunger-to-barrel contact depends on the spring bias, fit tolerances of the plunger and the barrel, contact surface topography, and plating uniformity on the inside of the barrel. The diametrical clearance between the plungers and the barrel must be precisely controlled to prevent an excessive plunger tilt. Since the contact between the plunger and the bore of the barrel is localized along the line of contact where the edge of the plunger traverses the bore of the barrel, an excessive tilt often causes accelerated wear of contacting surfaces and a premature contact probe failure.
In the case of miniature sockets, contact compliance is an important consideration. The contact probe must have and adequate compliance (deflection capability) to account for planarity tolerances (z-directions variation of I/O terminal location due to tolerances, bowing of the board, etc.,), and to provide an adequate contact engagement even in worst case I/O terminal positioning. However, miniaturization of contact probes often leads to compliance problems since the small probes tend to have a low deflection capability, while the manufacturing tolerances, assembly tolerances, and board planarity, do not scale accordingly and remain substantially the same as for larger contacts.
Most test probes rely on a coil spring to provide the contact force and the resilient compliance necessary to assure that the contact force is in the desired range in the worst case tolerance conditions. Coil springs are relatively easy to manufacture in varying sizes, configurations, materials, and degree of compliance. In addition to providing a contact force, a compression spring may also serve as a conductive member. However, a coil spring acts as an electrical inductor at high frequencies and therefore presents electrical performance problems. Furthermore, it is often desirable to make the spring from a music wire or stainless steel which have poor electrical conductivity. This conflict between electrical and mechanical performance of the spring is best resolved by making the spring electrically insignificant. Various mechanisms have been employed to mitigate the adverse electrical effects of the spring, as illustrated by the patents cited below.
Another important characteristics of contact probes is the pointing accuracy, i.e., the ability to center the tip of the plunger on the target I/O terminal. The pointing accuracy becomes more important with dense trace patterns and smaller contact targets of highly integrated circuits. On the other hand, the pointing accuracy becomes more difficult to control in small contact probes as the plunger-to-barrel diametrical clearance is likely to have more impact on tilting of the plunger, especially when the plunger bodies which are guided in the barrel become very short. While the random tilting of the plungers enables the plunger-to-barrel contact, it adversely affects the probe's pointing accuracy.
Still another important consideration in many contact probes is the impact of friction on the contact force and the useful life of the probe. While the spring characteristics is usually linear, the friction can lead to non-linear or erratic forces. A sliding contact between components will generate friction which must be overcome by the spring bias, which can reduce the primary contact forces between the plungers and the respective I/O terminals. In the case of the tilted plungers in sliding contact with the bore, the bore surface is often irregular and the contact force between the edge of the plunger and the bore of the barrel is difficult to control. In severe cases, gauging of surfaces may expose bare metal, cause oxidation of surfaces and accumulation of a nonconductive debris between contact surfaces. This will cause high contact interface resistance and/or high friction forces which can cause a plunger to seize in the bore.
In view of the above, plating of contact probe components is an important factor in performance and useful life of contact probes. A barrier metal such as nickel and a noble metal such as gold are typically applied to the surfaces of the contact components to assure a low contact interface resistance and to protect contacts from corrosion. Plating of high aspect ratio and small diameter barrels creates challenges due to the difficulty of circulating the plating solution through a small bore. On the other hand, the bore of the barrel needs adequate plating to allow the plunger to make a reliable conductive connection to the surface of the bore. The need to assure an adequate plating thickness inside the barrel causes wasteful plating of non-contact outside surfaces of the barrel; up to 1.0 um of gold may be applied to the outside of the barrel in order to assure 0.5 um min plating inside the barrel, where conductive contact between the plunger and the barrel is made. Furthermore, the in-process quality control of the plating inside the barrel is difficult as the plating uniformity and thickness can only be evaluated by cutting the barrel lengthwise to expose the bore. In addition, the base surface of the bore is often irregular and difficult to clean thoroughly prior to plating. Plunger plating is also critical to the probe's electrical performance and often requires a hard, wear-resistant plating with good coverage of sharp points and edges.
The probe's cycle life is an important consideration in many test applications. If the probe length is short, the probe design, materials, and contact forces have significant impact on the cycle life of the probe and tradeoffs are necessary. The material of choice for miniature springs (e.g., having a mean coil diameter of less than 1.0 mm and a wire diameter of 0.05 to 0.15 mm) is music wire. Music wire has a very high tensile strength, and can provide a long mechanical life at high operating stress. However, music wire is made from a high carbon steel, is magnetic, and has low electrical conductivity. On the other hand, the preferred material for a spring that is used as a conductive member is beryllium copper, which has a higher conductivity but a lower elastic modulus and a lower strength than music wire.
While very reliable and durable probes can be produced in larger sizes, the desired probe characteristics is difficult to achieve in miniature probes, e.g., probes having an outside diameter of less than 1.0 mm and the length of less than 5 mm. In view of the machining tolerances and plating thickness variation on the inside of the barrel and on the plunger, the diametrical fit between the plunger and the barrel is difficult to tightly control. Since the plunger bodies which are guided in the bore of the barrel tend to be short, even a small diametrical clearance between the plunger and the bore can cause an excessive tilt of the plunger. The excessive plunger tilt can lead to a high localized wear and increased friction which may cause seizing of the plunger in the barrel, preventing return of the plunger to the original extension, and thus causing a premature contact failure. The surface defects and plating irregularities can significantly affect the sliding friction. The short probes also tend to have stiffer springs and less compliance. This can lead to large variation in contact forces and cause damage to the I/O terminals (e.g. deformation or shearing of BGA solder balls) when the contact forces are excessive.
Contact probes have been proposed to address some of the above issues as illustrated by the following patents:
U.S. Pat. No. 7,535,241 (2009) to Sinclair discloses a contact probe having a barrel, a coil spring, and a single plunger. The barrel has a stepped closed end which serves as a stop for the spring and allows the plunger body to conductively short to the barrel. This contact probe relies on a random tilting of the plunger for contact with the inside surface of the barrel. The bottom of the barrel must be reliably plated, which is difficult, especially when small diameter, large aspect ratio barrels are used. The deep drawn barrel can only provide an integral bull-nosed contact since pointed tips cannot be produced by the deep drawing process.
U.S. Pat. No. 5,990,697 (1999) to Kazama discloses a contact probe which utilizes a variable pitch coil spring as a primary conductive element. Such spring would be typically made from a higher conductivity alloy such as beryllium copper. The contact has some closely wound coils that become conductively shorted as the deflection progresses. Other coils must remain active so that a solid height is not reached. In order to satisfy the compliance requirement, these springs still require a substantial number of coils which are initially open, and only progressively are being closed (shorted) as the spring is being compressed. Such springs have a non-linear force vs deflection characteristics and can introduce a substantial variation in contact force and inductance due to manufacturing tolerances and non-planarity of mating interfaces. In worst tolerance cases, at a maximum deflection condition the contact force can be excessive, while at a minimum deflection condition an insufficient number of coils may be shorted so that the inductance can be excessively high.
U.S. Pat. No. 7,019,222 (2006) to Vinther discloses a one-piece coil spring contact wherein the coils are at an oblique angle to the direction of compression and are conductively shorted when the spring is compressed. While such contact can provide an excellent electrical performance, it is not scalable to smaller sizes without a significant loss of compliance (deflection capability). In this case, increasing compliance by increasing the number of coils will lead to a wider contact and will necessitate larger contact-to-contact spacing. Furthermore, the contact is not easily adaptable for use with a variety of plungers which are often needed to adapt the contacts to a particular I/O terminal configuration, such as a solder ball of a BGA device. In contrast, the conventional coil spring contacts are generally scalable to denser grids by extending the length when the spring diameter is reduced. (Although this quickly leads to excessively long contact probes with a high self-inductance.)
Other examples of low inductance contacts and probes can be found in U.S. Pat. Nos. 7,556,503 (2009) to Vinther; 7,134,920 (2006) to Ju et al; 6,696,850 (2004) to Sanders; 6,666,690 (2003) to Ishizuka et al; 6,043,666 (2000) to Kazama; 6,033,233 (2000) to Haseyama et al; and 5,641,314 (1997) to Swart et al.
The recent increases in circuit integration and operating frequencies pushed the available probes and contacts to their performance limits. While particular known contacts and probes have addressed some of the needs with various degree of success, none combine all the desired features in a single design. Consequently, there is a need for improved miniature contact probes and contacts having low contact inductance, low contact resistance, and which are suitable for use in test sockets and connectors with close contact spacing and high contact count.